1. Field of the Invention
The invention refers to a leak detector with optical tracer gas detection.
2. Description of the Prior Art
Leak detectors typically include a mass spectrometer or a similar gas analysis apparatus to examine a gas drawn in for the presence of a tracer gas. Tracer gases of choice are helium and other noble gases as well as hydrogen. The use of a mass spectrometer requires the generation of a high vacuum which in turn demands for bulky vacuum pumps. Further, leak detectors are known that have a cell which comprises gas-tight walls and is closed with a membrane selectively permeable to the tracer gas, where the membrane forms a tracer gas inlet. Within the cell, the partial pressure of the tracer gas rises if the tracer gas is present at the membrane outside the cell. Since the cell holds no other gases than the tracer gas, the partial pressure of the tracer gas can be measured in the cell by means of a total pressure measurement. This gives information about the tracer gas partial pressure in the ambience. Thus, it is not only possible to detect the presence of tracer gas in the environment, but quantitative measurements are also possible. Pressure measurement inside the cell requires an intricate measuring device and a pumping function for the removal of the tracer gas. Penning or magnetron cells are suitable cells for this purpose.
DE 198 53 049 C2 describes another type of leak detector wherein a carrier gas is pumped through the object under test and wherein it is detected whether a tracer gas is present in the outflowing carrier gas. If this is the case, a leak in the object under test has been determined. The gas pumped from the object under test is passed through a discharge cell and caused to assume a metastable state. The carrier gas or tracer gas of choice for generating the gas discharge is helium. It is the purpose of the gas discharge to cause the tracer gas to assume the metastable state. The discharge cell includes an optical measurement path formed by a laser and a photo detector receiving the beam from the laser. The excited atoms of the tracer gas or of the tracer gas component to be detected are measured in the discharge cell by means of laser absorption spectrometry. This measuring principle requires that the carrier gas necessary for the excitation of the tracer gas be passed through the object under test.